The present invention relates generally to programmable implantable pacemakers, and more particularly, to a system and method for preventing atrial competition in an implantable dual-chamber pacemaker programmed to operate in a sensor-driven mode, i.e., in a mode wherein a physiological sensor provides an indication of the rate at which the pacemaker should provide pacing pulses on demand.
A brief review of cardiac physiology and pacemaker technology will first be presented to help better understand the present invention and the terminology used herein.
The heart is a pump which pumps blood throughout the body. It consists of four chambers, two atria and two ventricles. In order to efficiently perform its function as a pump, the atrial muscles and ventricular muscles must contract in a proper sequence and timed relationship.
In a given cardiac cycle (corresponding to one "beat" of the heart), the two atria contract, forcing the blood therein into the ventricles. A short time later, the two ventricles contract, forcing the blood therein to the lungs (right ventricle) or through the body (left ventricle). Meanwhile, blood returning from the body fills up the right atrium and blood returning from the lungs fills up the left atrium, waiting for the next cycle to begin. A typical healthy adult heart may beat at a rate of 60-70 beats per minute (bpm) while at rest, and may increase its rate to 140-180 bpm when the adult is engaging in strenuous physical exercise, or undergoing other physiologic stress.
The healthy heart controls its own rhythm naturally from its sinal-atrial (S-A) node, located in the upper portions of the right atrium. The S-A node generates an electrical impulse at a rate commonly referred to as the "sinus" rate. This impulse is delivered to the atrial tissue when the atria are to contract; and, after a suitable delay (on the order of 120-180 milliseconds), is delivered to the ventricular tissue when the ventricles are to contract.
When the atria contract, a detectable electrical signal referred to as a P-wave is generated. When the ventricles contract, a detectable electrical signal referred to as an R-wave is generated. The R-wave is much larger than the P-wave, principally because the ventricular muscle tissue is much more massive than is the atrial muscle tissue. The atrial muscle tissue need only produce a contraction sufficient to move the blood a very short distance, from the respective atrium to its corresponding ventricle. The ventricular muscle tissue, on the other hand, must produce a contraction sufficient to push the blood over a long distance, e.g., through the complete circulatory system of the entire body.
Other electrical signals or waves are also detectable within a cardiac cycle, such as a Q-wave (which immediately precedes an R-wave), an S-wave (which immediately follows an R-wave), and a T-wave (which represents the repolarization of the ventricular muscle tissue).
A pacemaker is a medical device that provides electrical stimulation pulses to the appropriate chamber(s) of the heart (atria or ventricles) in the event the heart is unable to beat on its own, i.e., in the event either the S-A node fails to generate its own natural stimulation pulses at an appropriate sinus rate, or in the event such natural stimulation pulses are not delivered to the appropriate cardiac tissue. Most modern pacemakers accomplish this function by operating in a "demand" mode wherein stimulation pulses from the pacemaker are provided to the heart only when it is not beating on its own, as sensed by monitoring the appropriate chamber of the heart for the occurrence of a P-wave or an R-wave. If a P-wave or an R-wave is not sensed within a prescribed period of time (which period of time is most often referred to as the "escape interval"), then a stimulation pulse is generated at the conclusion of this prescribed period of time and delivered to the appropriate heart chamber via a pacemaker lead.
Further details associated with cardiac physiology and the operation of the heart as controlled or monitored by a pacemaker may be found, e.g., in U.S. Pat. No. 4,712,555 to Thornander et al.; U.S. Pat. No. 4,788,980 to Mann et al.; and/or U.S. Pat. No. 4,944,298 to Sholder. All three of these patents are incorporated herein by reference.
Pacemakers are typically both implantable within a patient and programmable, allowing their operation to be selectively controlled from a location external to the patient. Modern programmable pacemakers are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. The present invention relates to dual-chamber pacemakers, and more particularly to dual-chamber pacemakers operating in a rate-responsive mode.
In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and/or senses cardiac activity within, a single-chamber of the heart, e.g., either the right ventricle or the right atrium. In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and/or senses cardiac activity within, two chambers of the heart, e.g., both the right ventricle and the right atrium. Typically, only the right atrium and/or the right ventricle is coupled to the pacemaker because of the relative ease with which a pacing lead can be transvenously inserted into either of these chambers. However, the left atrium and left ventricle can also be paced just as effectively providing that suitable electrical contact is made therewith.
In general, both single and dual-chamber pacemakers are classified by type according to a three or four letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a "V" indicating the ventricle, an "A" indicating the atrium, and a "D" indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters to identify the atrium or ventricle or both, and wherein a "0" indicates no sensing takes place.
The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting ("I") response wherein a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger ("T") response wherein a stimulation pulse is delivered to a prescribed chamber of the heart a prescribed period after a sensed event; (3) or a Dual ("D") response wherein both the Inhibiting mode and Trigger mode are evoked, e.g., inhibiting in one chamber of the heart and triggering in the other.
The fourth letter, when used, indicates whether the pacemaker is operating in a sensor-driven mode, i.e., in a mode wherein a physiological sensor is used to provide an indication of what the rate of the pacemaker should be. Such rate is often referred to as the sensor-indicated rate (SIR). The letter "R" is frequently used for the fourth letter to indicate use of such a sensor-driven mode.
Thus, for example, a DVI pacemaker is a pacer (note that throughout this application, the terms "pacemaker" and "pacer" may be used interchangeably) that paces in both chambers of the heart, but only senses in the ventricle, and that operates by inhibiting stimulation pulses when prior ventricular activity is sensed. Because it paces in two chambers, it is considered as a dual-chamber pacemaker. A VVI pacer, on the other hand, is a pacer that paces only in the ventricle and senses only in the ventricle. A VVIR pacer is a pacer that paces only in the ventricle at a rate determined by an appropriate physiological sensor, and senses only in the ventricle. Because only one chamber is involved, a VVI or VVIR pacer is classified as a single-chamber pacemaker.
Most dual-chamber pacemakers can be programmed to operate in any desired mode, including a single-chamber mode. Hence, e.g., a dual-chamber pacemaker may be programmed to operate in a DDD mode, i.e., a mode wherein the pacemaker paces and senses in both the atrium and the ventricle. If the dual-chamber pacemaker includes a physiological sensor, the dual-chamber pacemaker may be programmed to operate in a DDDR mode, i.e., a mode wherein the pacemaker provides stimulation pulses to both chambers of the heart on demand (i.e., only in the absence of natural atrial or ventricular activity in the respective chambers, as determined by sensing in both chambers) at a rate determined by the physiological sensor. The present invention addresses a problem that is primarily associated with a dual-chamber pacemaker operating in a DDDR mode.
One possible effect caused by operating a pacer in a DDD mode is atrial rate based pacing. In an atrial rate based pacemaker, the rate of the pacemaker is set by the heart's S-A node, and the ventricle is paced at a rate following the sensed atrial rate. Because the rate set by the S-A node represents the rate at which the heart should beat in order to meet the physiologic demands of the body, at least for a heart having a properly functioning S-A node, the rate maintained in the ventricle by such a pacemaker is truly physiologic. As indicated, a dual-chamber pacemaker, programmed to operate in the DDD mode, provides such physiologic pacing. That is, one of the functional states of DDD pacing, particularly applicable to patients having A-V block, is to sense P-waves in the atrium, i.e., to sense the rate set by the S-A node, and pace the ventricle at such sensed rate. Thus, as the physiologic rate increases, e.g., as the patient exercises and the P-wave rate increases, the pacemaker is able to track such increase and pace the ventricle accordingly.
Unfortunately, in a conventional DDD pacer, P-waves are tracked only up to a certain limit. If the P-waves occur too rapidly, they begin to fall in what is known as the atrial refractory period (ARP), the relevant portion of which is often referred to as the post ventricular atrial refractory period (PVARP) because it occurs after ventricular activity, whether such ventricular activity is paced or sensed. During the atrial refractory period, which is a prescribed time period set by the pacemaker logic circuits, P-waves are not sensed; or, if they are sensed, they are not considered as a P-wave, but are rather considered as noise. P-waves that occur during the PVARP thus have no effect on pacer timing. The PVARP is intended to provide a sufficient waiting period for the heart tissue to settle down or recover following a prior depolarization or contraction. (See, e.g., the previously cited '555 patent, and/or the '980 patent, for a more complete description of the timing intervals, and time periods, measured and/or generated by a typical pacemaker as it performs its function of providing stimulation pulses on demand.)
Thus, if the rate at which P-waves occur increases sufficiently to place a P-wave within the PVARP, such P-wave is not detected by a DDD pacer, and the occurrence of such P-wave has no effect on pacer timing. That is, the DDD pacer has no way of knowing that the P-wave occurred, so it waits until the next P-wave occurs, or until the pacemaker's applicable escape interval times out, whichever occurs first, before initiating the appropriate mechanism for issuing a ventricular stimulation pulse ("V-pulse"). Disadvantageously, for a situation where the intrinsic P-waves are gradually increasing, each being followed by a V-pulse, a point is reached (when the P-wave enters the PVARP) where the intrinsic P-wave is not sensed, resulting in an abrupt decrease in the ventricular paced rate.
In order to overcome this difficulty--of an abrupt decrease in the ventricular rate when tracking P-waves that enter the PVARP--it is known in the art to utilize a DDDR pacing mode. See Hanich et al., "Circumvention of Maximum Tracking Limitations with a Rate Modulated Dual-chamber Pacemaker," PACE 12:392-97 (February 1989). Such DDDR pacing mode offers the advantage of providing a sensor-indicated back-up pacing rate after the intrinsic P-waves enter the PVARP. Thus, an abrupt decrease in the ventricular paced rate is avoided because the applicable escape interval in such a rate-responsive pacemaker, e.g., a DDDR pacemaker, is adjusted automatically as a function of the sensor-driven rate. Hence, as the intrinsic P-wave rate increases due to increased physiological demand brought about by, e.g., exercise, the applicable escape interval is shortened by the sensor-driven rate. Thus, even though a P-wave may enter the PVARP and not be sensed, the pacemaker will soon issue an atrial stimulation pulse, followed by a V-pulse, at a rate determined by the sensor-driven rate, thereby avoiding abrupt changes in ventricular paced rate.
Disadvantageously, however, once detection of the intrinsic P-wave is lost due to its falling within the PVARP, the resulting atrial stimulation pulse ("A-pulse") occurring at the sensor-indicated rate is in competition with the P-wave. Such atrial competition is undesirable because it may induce, in many patients, atrial arrhythmias. This is especially true in those instances where the patient's intrinsic atrial rate has increased due to increased physiological demand, as during physical exercise, because during such times the heart is experiencing higher myocardial oxygen demand and may be experiencing relative ischemia (inadequate flow of blood), both of which conditions may further promote the atrial arrhythmia.
An atrial arrhythmia, if it is short lived, is usually of no consequence. However, if it persists, it may result in an atrial tachycardia (a very rapid atrial rhythm) or fibrillation, both of which conditions pose serious health risks to the patient. Hence, what is needed is a method or technique for preventing atrial competition in a patient having a DDDR pacer, particularly when the DDDR pacer is sensing and tracking intrinsic P-waves that fall into the PVARP of the pacemaker.
Atrial competition also creates other problems. For example, atrial competition, by definition, applies an atrial stimulation pulse to atrial tissue as it is repolarizing (i.e., shortly after contraction). This action can significantly desensitize the atrial tissue to subsequent stimulation pulses, thereby making it difficult to achieve and maintain "capture" at those times when capture is needed to maintain a desired pacing rate. ("Capture" refers to the response of cardiac tissue to an applied stimulation pulse. When of sufficient energy, a stimulation pulse causes cardiac tissue to which it is applied to depolarize and contract; and the cardiac tissue is said to be "captured" by the stimulation pulse. When of insufficient energy, the cardiac tissue does not depolarize and contract; and the cardiac tissue does not respond, i.e., is not captured, by the stimulation pulse.) Thus, applying an A-pulse to the atrium in competition with a P-wave may make subsequent atrial capture difficult to achieve, as well as introduce atrial arrhythmias. Thus, what is needed is a system and method for operating a DDDR pacer wherein lack of capture is avoided, and atrial arrhythmias are prevented.
Further, should an atrial arrhythmia persist, there is a need in the art for a method or technique for quickly terminating such arrhythmia. However, before such terminating technique can be invoked, there is also a need for a reliable technique for distinguishing between a true atrial arrhythmia (a potentially dangerous cardiac condition) and a fast atrial rate (which may be a normal and needed response to a high oxygen demand condition, such as exercise).
Also, it is noted that as a practical matter atrial competition (i.e., the generating of an A-pulse in competition with an intrinsic P-wave) represents wasted energy of the limited energy resources of the implanted pacemaker. That is, the atrium, having just naturally depolarized and contracted, is not capable of depolarizing and contracting again until such time as the atrial cardiac tissue has repolarized. Thus, there is a further need to avoid atrial competition in order to preserve the limited energy resources of the pacemaker.
Advantageously, the dual-chamber, sensor-driven pacemaker described herein, including the method of operating such pacemaker, addresses the above and other needs.